LONDON – British police questioned 22 suspects Sunday in the alleged plot to bomb trans-Atlantic jetliners in mid-air, and the country's top law-and-order official warned that the risk of another attack still remained high.

Home Secretary John Reid said the threat "is a chronic one and it is a severe one."

Reid repeated the assertion, made before by police, that Britain has foiled four major terrorist plots since the July 7, 2005, London transit bombings, and said police were conducting about 24 anti-terrorist investigations.

Britain's terrorist threat level remained "critical" — its highest designation — and delays, flight cancellations and intense security continued to greet many travelers at London airports.

"We believe we have the main targets from this particular surveillance and plot," Reid told British Broadcasting Corp. television. "[But] there are still people out there who would carry out such attacks."

Police arrested 24 people across England on Thursday, saying they had thwarted a plot to blow up as many as 10 passenger planes flying between Britain and the United States. One suspect was released without charge, and a court will decide Monday on the detention of another. The suspect cannot be questioned in the interim.

The rest — most young British-born Muslims — were being questioned at high-security police stations. Under tough new anti-terrorism laws, authorities can hold suspects up to 28 days before they must be charged or released.

Among the leads British police were following was whether any of the suspects have connections to the 2005 London transit suicide bombers, or whether any others among them visited Pakistan in recent months. They are also examining Internet cafes near the suspects' homes with the possibility of tracking Web based e-mails or instant messages.

The international investigation has zeroed in on brothers Rashid and Tayib Rauf, the former arrested in Pakistan, the latter in Britain. Their father, Abdul Rauf, immigrated to Britain from the Mirpur district of Pakistan several decades ago, and his five children were all born in Britain.

Rashid Rauf was arrested about a week ago along the Pakistan-Afghan border, and Pakistani officials have named him as a "key person." They say there is evidence he was linked to an "Afghanistan-based Al Qaeda connection" but have given no details.

His 22-year-old brother Tayib was taken into custody in Britain, and there were unconfirmed reports a third brother may have been detained.

Relatives and other neighbors of the Raufs expressed shock that they were caught up in the inquiry, but the family is no stranger to the authorities. The Raufs' terraced home was first searched during a 2002 investigation into the fatal stabbing of Mohammed Saeed, the Rauf brothers' uncle, police said.

Rashid Rauf was reportedly a suspect in that slaying, and is believed to have left England for Pakistan shortly after Saeed's death. The home was searched again in connection with a murder during race riots in 2005.

Pakistan is questioning at least 17 people, including Rauf and one other British national whose name has not been released. A senior Pakistani security official told The Associated Press that Rauf's arrest prompted an accomplice in the southern city of Karachi to make a panicked phone call to a suspect in the United Kingdom, giving the green-light for the airliner plot to move forward urgently.

"This telephone call intercept in Karachi and the arrest of Rashid Rauf helped a lot to foil the terror plan," the official said.

A second intelligence official described the accomplice as "inexperienced," and said the caller "alerted his associates about the arrest of Rashid Rauf, and asked them to go ahead." Both officials spoke on condition of anonymity due to the sensitive nature of their work.

While authorities in Pakistan believe they have nabbed the main players in the plot, the official said there are two or three people still at large, including Matiur Rahman, a senior figure in the Al Qaeda-linked Pakistani militant group Lashkar-e-Jhangvi. Rahman's name was mentioned by one of the detainees during interrogation.

Many in Britain's Muslim community are deeply distrustful of the police following high-profile blunders in the past, including the killing of a man mistaken for a suicide bomber and the shooting of another man in a raid that resulted in no charges.

Prominent British Muslims, including three members of Parliament, also complained in an open letter Saturday that Britain's intervention in Iraq and the failure to secure an immediate cease-fire between Israel and Hezbollah were providing "ammunition to extremists who threaten us all."